1. Field of the Invention
The invention relates to an automatic transmission control device in which gears of the transmission are shifted or changed by a plurality of friction engaging elements.
2. Description of the Related Art
Shift control systems of an automatic transmission are operated, on the one hand, by a combination of one-way clutches, multiple friction engaging elements (clutches, brakes) and band brakes and, the other hand, by a combination of multiple friction engaging elements (clutches, brakes). The shift control system includes a timing control for timing the intake and exhaust of the hydraulic operating oil supply from an operating means of a friction engaging element. The timing control can operate to exhaust the hydraulic operating oil when the oil pressure on the intake side of the operator of the friction element reaches a predetermined pressure value. This type of a timing control is operated by, for example, a shift timing valve shown in a manual for the Toyota Landcruiser New Automobile, lines 10-36, and FIG. 6 (published in 1984). The timing valve therein is a 2.fwdarw.3 shift timing valve which controls timing for draining the oil pressure of the brake B1 when the system is shifted from the 2nd gear to the 3rd gear so as to disengage the brake B1 and engage the clutch C3. The timing valve reduces the shift-shock which occurs during the shift between gear ratios. In the 2.fwdarw.3 shift timing valve, a spool 1 is urged to the upper position by the spring 2 while the vehicle is operated in the second gear. When changing to the third gear, the engaging oil pressure P.sub.c2 of the rear clutch C.sub.2 is transmitted through an orifice (not shown) from the 2.fwdarw.3 shift valve via passage L.sub.1. The spool 1 is then urged to the lower position against the oil pressure P.sub.ACC transmitted from the accumulator control valve through the passage L.sub.2 and against the force of the spring 2. When the spool 1 is moving to the lower position and the oil pressure P.sub.C2 does not increase sufficiently, the oil pressure P.sub.B1 of the brake B1 transmitted through the passage L.sub.3 only passes through the passage L.sub.4 and the orifice 3. Therefore, the engagement of the second brake B1 is maintained. When the engaging oil pressure P.sub.C2 of the clutch C.sub.2 increases further and the spool 1 moves further to a lower position, the passage L.sub.5 provided below the orifice 3 is opened and the oil pressure P.sub.B1 is quickly reduced and the engagement of the brake B1 is released. At this time, the oil pressure of the rear clutch C2 is changed to a pressure, the range of which overlaps slightly with the pressure range of the second brake B1, and the rear clutch C.sub.2 is engaged. Thereby, the shifting to the third gear ratio is completed.
In the up shift of the automatic transmission, there is a power-off up-shift and a power-on up-shift. The power-off up-shift, using a combination of the multiple friction engaging elements employs an underlap control to quickly release the engaging element of the disengaging side and to slowly engage the engaging element of the engaging side, as shown in FIG. 7 by a full Line .alpha.. When the overlap control, shown in FIG. 7 by a dotted line .beta., is carried out at the power-off up-shift, the throttle pressure is in a reduced condition and the change of the output torque of the overlap control shown by the dotted line .delta. of FIG. 8 is greater than the one of the underlap control shown by the full line .gamma., therefore the shift shock also becomes greater.
In the above-mentioned conventional shift timing control at the up-shift, the change of the shift timing valve is performed in relation to the accumulator back pressure modulated by the engaging oil pressure of the engaging side and the throttle pressure and is not affected by the speed of the automobile. During the up-shift either the underlap control or the overlap control is selected dependent on whether the degree of throttle opening is within a certain range. In fact, however, not only the degree of throttle opening but also the automobile speed (the output revolution) is decisive.
For example, when the degree of throttle opening is large enough during the power-off up-shift, the underlap control is not required. By contrast when the degree of throttle opening is not large enough in the power-on up-shift, the underlap control is required and the degree of throttle opening is different in relation to the automobile speed.
Therefore, in the above-discussed conventional shift timing control, the shift shock is not sufficiently reduced.